The invention relates to a reference electrode comprising an electrode plug and a chamber closed at the rear containing an electrolyte, the chamber having an aperture covered by a membrane of a material permitting diffusion of ions therethrough, but whose surface is closed for proteins and similar macromolecular substances.
Reference electrodes are used as part of an electrode chain when electrochemically measuring the content of a particular chemical substance (species) in a liquid sample. The reference electrode establishes a constant or stable potential, which in an ideal situation is independent of the composition of the liquid sample, but in practice varies with the so-called liquid junction potential. The liquid junction potential is the potential difference, created across the interface between the sample liquid and electrolyte, and this potential varies with varying dilution and varying ion composition between sample and electrolyte. This variation affects the measuring results which become imprecise or misleading.
When using a reference electrode of the present kind, it is decisive that the concentration of the potential controlling ion in the electrolyte is constant or at least kept within certain limits in order to obtain stability and thus to achieve reliable measurements. It is known to fulfil this requirement either by using a saturated solution as electrolyte, which, however, is not always possible due to practical/technical causes, or by using a system where the electrolyte is renewed after each measurement which, for example, is the case at a so-called open salt bridge system. Without renewal of the electrolyte, the diffusion from the sample to the electrolyte may change the composition of the electrolyte and thus change the reference electrode potential.
A known reference electrode, E111 Reference Electrode sold by the company Radiometer Medical A/S, {dot over (A)}kandevej 21, DK-2700 Brxc3x8nshxc3x8j, Denmark, comprises an electrode plug and a chamber closed at the rear containing a saturated KCl solution. The chamber has an aperture covered by a porous double membrane which permits diffusion of ions therethrough both radially and across the membrane. The porous membrane comprises of a 12 xcexcm thick polycarbonate layer and a 18 xcexcm thick cellophane layer. The saturated KCl solution diffuses through the porous membrane which serves as salt bridge, thus establishing contact between the reference electrode and the sample. This known electrode suffers from the drawback that liquids in the measuring chamber pollutes the liquid junction, and thus reduces the use of the electrode.
Another known reference electrode is used in an analyser, Chiron 800 series, which is manufactured and sold by the company CHIRON DIAGNOSTICS CORPORATION, Ameryville, Calif., USA. This electrode also comprises a chamber containing an electrolyte and having an aperture covered by a membrane which permits ion diffusion. However, in this electrode the chamber is open at the rear. The following FIG. 1 shows an outline of the area at the aperture and the membrane of this known reference electrode. The membrane is of cellophane and is fixed between two walls having concentric apertures, of which the inner aperture relative to the chamber is approx. 0.7 mm in diameter and the outer aperture is approx. 0.8 mm in diameter.
In the latter electrode there is an actual flow of electrolyte out through the membrane, in reality, the electrode uses an open salt bridge system. This raises the problem that the electrolyte penetrates into the liquid sample being measured, which is polluted and subsequently cannot be used for other measurements. Besides, the use of an open salt bridge system requires that the electrolyte is continuously being supplied.
The purpose of the invention is to provide a reference electrode which compared to the prior art has reduced the exchange of ions between the electrolyte and the sample liquid.
This is achieved by means of a reference electrode comprising an electrode plug and a chamber closed at the rear and containing an electrolyte, the chamber having an aperture covered by a membrane of a material permitting diffusion of ions therethrough, but whose surface is closed for proteins and similar macromolecular substances, and which is characterised in comprising a diffusion tight diaphragm element having a radial through-going orifice situated within the aperture, which diaphragm element is situated on the outer side of the ion diffusion membrane relative to the chamber, and which orifice is equal to or smaller than the aperture, thus limiting variations in the liquid junction potential.
Since the aperture in the chamber is greater than the orifice in the diaphragm element, ions form the electrolyte in the chamber are supplied to the orifice area of the ion diffusion membrane from all directions both axially and radially, whereas ions from the sample liquid may only approach the orifice area of the ion diffusion membrane in an axial direction at the specific area of the orifice. Thus, the diffusion of ions from the electrolyte to the ion diffusion membrane is greater than the diffusion of ions from the sample to the ion diffusion membrane. Therefore, ions from the sample liquid do not pollute the electrolyte by diffusing into the electrode chamber and the electrolyte remains present in full concentration in the chamber. As a result, it is not necessary to supply further electrolyte to the chamber during the life time of the electrode.
Theoretically, the reference electrode should operate satisfactorily having almost the same orifice size in the diaphragm element as the aperture, but according to tests made by the inventors, the diaphragm element should preferably cover at least 80-90% and preferably at least approx. 93% of the area of the aperture, corresponding to the area of the orifice being max. 10-20% and preferably max. 7% of the area of the aperture, and/or the orifice should preferably have a diameter of max. 0.4 mm, more preferably max. 0.25 mm.
A reduction of the size of the orifice relative to the size of the aperture improves the radial supply of ions from the chamber to the orifice through the ion diffusion membrane.
A reduction of the absolute size of the orifice reduces the exchange of ions between the electrolyte and the sample liquid, resulting in only an insignificant change in the concentration of ions in the electrolyte and in particular of the potential determining ion during use.
The diameter of the orifice should be as small as possible when measuring pure liquids without dissolved macromolecular compounds. However, when measuring for example blood or another liquid containing proteins or other macromolecular substances the diameter should not be so small that there may be a risk of wholly or partly clogging. Therefore, the diameter of the orifice is preferably greater than 0.05 mm.
In a preferred embodiment, the orifice has a diameter in the area of approx. 0.12-0.25 mm, and a nominal diameter of 0.18 mm.
The diaphragm element preferably is a membrane in order to achieve a small thickness of the element, thereby avoiding that special diffusion conditions in the orifice arises, which may affect the potential. The thickness of the element to be used depends on the size of the orifice. The orifice diameter of the diaphragm may be adjusted depending on the character of the sample liquid, as relatively large orifice diameters are used (larger than approx. 0.05 mm) when measuring sample media having protein content such as blood. The thickness of the diaphragm membrane is preferably in the interval 5-25 xcexcm. The ratio between the orifice diameter and the membrane thickness of the diaphragm is preferably within the interval 0.008-100. In a preferred embodiment the diaphragm membrane is of polyester and the ratio between the orifice diameter and the thickness of the diaphragm membrane is 12 (0.18 mm/15 xcexcm).
In a further preferred embodiment, the diaphragm element is made of polyester, but it may be made of any material which is tight against the used electrolyte and the occurring sample liquids, and which may be processed as required by the actual manufacturing method of the reference electrode, including the preparation of an orifice in it.
Further, from DE-A-37 24 040 is known a membrane unit (membrankxc3x6rper) for a membrane covered measuring electrode (a modified Clark-electrode) for measuring oxygen concentration in a fluid. The active surface of the membrane unit is limited by a diaphragm comprising an orifice which lies over the measuring electrode and preferably has a smaller diameter than this. The measuring electrode includes a chamber containing an electrolyte, and a membrane being impenetrable to the fluid and the electrolyte, but oxygen penetrable is stretched across the chamber. The orifice diaphragm limits the diffusion of oxygen from the fluid to the measuring electrode and side diffusion is excluded. This design ensures stabile and reproducible oxygen measurements. Thus, DE-A-37 24 040 relates to a substantially different electrode with a substantially different problem than the present electrode.
In a preferred embodiment of the reference electrode according to the invention the ion diffusion membrane permits three-dimensional diffusion of ions and is non-repellent for the electrolyte.
When using a water-based electrolyte, it is preferred that the ion diffusion membrane (preferably permitting three-dimensional diffusion) is of cellophane as by the above-mentioned known reference electrodes. However, synthetic or native materials may be used which may contain water in their polymer matrix. In these materials the water is either in direct contact with the polymer chains in a hydrated/swelled state due to polarity and free volume, or is contained in three-dimensionally combined pores or along hydrophilic fibres, which are distributed in the matrix, making tree-dimensional transport, penetration or diffusion of water possible. Mentioned among these materials may be chemical (cross-linked) or physical gels, for example the mentioned cellophane which is hydrophilic, but not water-soluble, cross-linked hydrophilic polymers (hydro gels), fibrous materials such as composites of the kind which are used in drain layers in disposable sanitary articles, cellular plastic (reticular, foaming) and heterogeneous compounds of polymers with mutually penetrating net.
Preferably a porous, mechanically strengthening membrane is located within the ion diffusion membrane. For example, this strengthening membrane may be of polycarbonate and be perforated with a pore diameter of approx. 0.1 xcexcm and a perforation degree of approx. 2.4%.
In a preferred embodiment, the electrolyte is a sodium format solution having a concentration in the area 1-8 M.
By means of the invention, it has been possible to develop a reference electrode consisting of replaceable units or a replaceable plug-in module. The reference electrode according to the invention may be shaped as an independent unit separate from a measuring electrode or constitute part of a unit integrated with a measuring electrode, i. e. be part of a so-called combination electrode.